Just thought I'd chime in, but Nikki's advice is great.
I have a short LP. Three cycles ago, it was only 5 days! I started taking B6/B12. I don't take a full B complex because my prenatal vitamin has so much of the other B vitamins that I really didn't want to overload my body with it. The B vitamins are water soluble, so you just urinate out what you don't need. B complex was turning my urine so dark, bright yellow that it was actually getting hard to read my OPK tests because they were stained. So I'm just taking 100mg B6 combined with 100mcg of B12, along with my prenatal every month. The B vitamins do work best when taken with each other, so if the B complex is working for you, stick with it. You might just increase your B6 a little bit on top of your current regimen.
I also wanted to just share with you my LP history. I was on BCP for almost 10 years, but I went off three years ago. We were not trying during that time, but the BCP was just wreaking havoc on my body after so long and I had to quit. When we started TTC our first cycle, I did nothing but time BD around ovulation, which I assumed was somewhere in the middle of my very consistent 30-day cycle. When we didn't get pg that cycle, I started charting. That's when I discovered the 5-day LP.
So for the next cycle, I started the B6/B12 regimen of 100mg. I O'd a day earlier and got pregnant. However, I believe due to the short LP, the egg couldn't latch on well and I miscarried. So I kept up with the B6, and last cycle I O'd 2 days earlier. When AF arrived, I discovered that I had a 10-day LP. So in just 3 months' time, I have extended it by 5 days. You really do just have to give it time. I honestly don't think that 100mg versus 200mg is going to make a huge difference. It's more the amount of time that you're taking it. I imagine you will see a great improvement this cycle.
As for other things that can help with LP, as Nikki mentioned, progesterone is something that a doctor may prescribe. They probably would not go that route, though, until you've charted several cycles. Short LP is often a progesterone deficiency, but not always. It can also be a problem with FSH, which stimulates the follicles and leads to ovulation. If you have an FSH problem, then it could be that your follicles are not maturing as they should. This leads to a shorter LP, even if you don't have a progesterone deficiency. Also, since the follicle is what becomes the corpus luteum, which produces progesterone during the first couple months' of a pregnancy, if the follicles are weak, the corpus luteum will be weak, and this can lead to not just a short LP but also chemical pregnancies/miscarriages. Again, your doctor probably won't go the route of testing your progesterone/FSH levels until you've been TTC for a few more cycles.
I very much understand the frustration you must be feeling, knowing how short your LP is. However, it is truly the most easily "fixable" fertility issue a woman can have. So I hope you take some comfort in that. Keep up with the B6/complex, and I bet you see better results this cycle.
One thing I forgot, some women take 81mg of aspirin (baby aspirin) throughout their cycle to promote blood flow to the uterus, which can help with short LP. Others drink red raspberry leaf tea (that's what I'm doing this cycle), which helps to tone the uterus. And still others use acupuncture to promote blood flow to the uterus. If we're not successful this cycle, I'm looking into that.
Try not to get too down. As I said, I bet the B vitamins will start helping a lot more in the coming month.